As an emerging technology, phase change materials attracted more and more interest for its applications in manufacturing a new type of highly intergrated nonvolatile memory devices, phase change random access memory (PRAM). PRAM uses the unique behavior of chalcogenide glass, which can be “switched” between two states, crystalline and amorphous, with the application of heat. The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity, and this forms the basis by which data are stored. The amorphous, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1.
Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. The stoichiometry or Ge:Sb:Te element ratio is 2:2:5. When GST is heated to a high temperature (over 600° C.), its chalcogenide crystallinity is lost. Once cooled, it is frozen into an amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a temperature above its crystallization point, but below the melting point, it will transform into a crystalline state with a much lower resistance.
One of the technical hurdles in designing PRAM cell is that in order to overcome the heat dissipation during the switching GST materials from crystalline to amorphous states at certain temperature, a high level of reset current needs to be applied. This heat dissipation can be greatly reduced by confining GST material into contact plugs. This would reduce the reset current needed for the action. Such a design, however, presents challenges for the techniques to form thin layer of GST materials. Since a plug structure is a high aspect ratio one, the conventional deposition method for GST films, or sputtering technique, due to its line-of-sight effect, has been found difficult to fill the plugs or high aspect ratio holes with GST materials. On the other hand, deposition techniques based on chemical reactions such as chemical vapor deposition (CVD) rely on transport and reaction of chemical vapors and do not have the line-of-sight effect. These techniques are better fits for such applications. In particular, the atomic layer deposition (ALD) can produce films with high conformality and chemical composition uniformity. Another technique in between ALD and CVD is so called cyclic CVD that may can be used for such applications.
To form a Ge—Sb—Te film that has a required stochiometry using CVD or ALD technique, one may need to form Ge, Sb or Te layer alternately followed by annealing. The thickness of each layer (Ge, Sb or Te) can be controlled so that a desired stochiometry ( e.g. 2,2,5 of Ge, Sb and Te) can be achieved in final product. Among these three layers, e.g., Ge, Sb and Te, Te may be the most difficult one to form. Currently available methods to form Te films either need high deposition temperature or plasma assist. However, either from materials and device performance or manufacturing cost, it is always preferred that the deposition can be performed at low temperature and without plasma assistance.
Hydrogen telluride has been used to prepare metal telluride, such as mercury telluride and cadmium telluride, as semiconductor materials. Dialkyltellurides are also used to make these materials.
Recently, the development of phase change memory has required the proper tellurium precursors for ALD or CVD deposition of GST films at relatively low temperature. Dialkyltelluride and diaminotellurides have been used. However, these precursors have low reactivity toward the deposition reaction. Sometimes it results low tellurium content in the films than required stoichiometrical composition.
Synthesis of bis(trialkylsilyl)tellurium has been reported, as has synthesis of other silyltellurium compounds: (Me3Si)3SiTeH.
This invention discloses methods to form thin Te films and GST films using chemical vapor deposition methods at low temperatures.